Gauge control method and apparatus including workpiece gauge deviation correction for metal rolling mills

ABSTRACT

A programmed computer control system provides on line roll force gauge control for a tandem hot steel strip rolling mill. An automatic gauge control system including a programmed digital computer calculates screwdown movement required for correction of determined gauge error on the basis of measured roll force and screwdown position values and on the basis of calculated gauge deviation corrections for the provided roll opening settings for the roll stands. To compensate for gauge error conditions, a gauge error correcting screwdown movement value is determined to establish the total amount of corrective screwdown movement required at any particular point in time. The control system operates the mill screwdowns in accordance with the program calculations.

United States Patent [191 Smith, Jr. et al.

[ GAUGE CONTROL METHOD AND APPARATUS INCLUDING WORKPIECE GAUGE DEVIATION CORRECTION FOR METAL ROLLING MILLS [451 Apr. 9, 1974 3,568,637 5/1971 Smith, Jr 72/8 3,574,280 4/1971 Smith. Jr

3,552,162 l/l97l Gingher, Jr. et a1 72/16 X Primary ExaminerMilton S. Mehr Attorney, Agent, or Firm-R. G. Brodahl 1571 ABSTRACT A programmed computer'control system provides on line roll force gauge control for a tandem hot steel strip rolling mill. An automatic gauge control system including a programmed digital computer calculates screwdown movement required for correction of determined gauge error on the basis of measured roll force and screwdown position values and on the basis of calculated gauge deviation corrections for the provided roll opening settings for the roll stands. To compensate for gauge error conditions, a gauge error correcting screwdown movement value is determined to establish the total amount of corrective screwdown movement required at any particular point in time. The control system operates the mill screwdowns in accordance with the program calculations.

13 Claims, 14 Drawing Figures [75] Inventors: Andrew W. Smith, Jr.; Richard Q.

Fox, both of Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Jan. 6, 1972 [21] Appl. No.: 215,749

[52] US. Cl. 72/11, 72/16 [51] Int. Cl B21b 37/00 [58] Field of Search 72/6l2, 16, 72/19, 21

[56] References Cited UNITED STATES PATENTS 3,332.263 7/1967 Beadle et al. 72/7 3,186,200 6/1965 Maxwell 72/8 3,232,084 2/1966 Sims 72/16 3,186,201 6/1965 Ludbrook et al. 72/9 3,631,697 l/l972 Deramo et al. 72/8 Ll SCREWDOWN POSITlONlNG CONTROL 33 3| m posmon osrecron l i 9 i 4 C l 23 l f t, some 65,120, r DETECTOR v l i 45 1;; i 1 CEL' g 53 S1 SPEED l erms on i i l -i fscRzwoowN posmonme l CONTROL l i 1-" "3) l ....In-. SCREWDOWN 5 POSITION I DETECTOR I SPEED l i 'T a TENSION CONTROL 1 DIGI AL i COMPUTER 1 "Mon mm] (WNYNOL PANl'l.

PAIENIEIJAPR FORCE SIIEU 02 min HD x SCREWDOWN POSITION OR LOADING ROLL OPENING HIN xIN FIG?) I I I I I S: O x- N IIIXIN SIO I SCREWDOWN POSITION OR LOADING ROLL OPENING (IN) a 5% 9ATENTEUAPR 9 m4 SHEET 03 0F 11 ENT GAU 53 54 STAND LOCATlON GAUGE CONTROL METHOD AND APPARATUS INCLUDING WORKPIECE GAUGE DEVIATION CORRECTION FOR METAL ROLLING MILLS CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to the following concurrently filed and related patent applicatiOnS WhlCl'l are assigned to BACKGROUND OF THE INVENTION The present invention relates to workpiece strip metal tandem rolling mills and more particularly to roll force gauge control systems and methods used in operating such rolling mills.

In the operation of a metal or steel reversing or tandem rolling mill, the unloaded roll opening and the speed at each tandem mill stand or for each reversing mill pass'are set up by the operator to produce successive workpiece (strip or plate) reductions resulting in work product at the desired gauge. Generally, the loaded roll opening at a stand equals the stand delivery gauge on the basis of the usually justifiable assumption that there is little or no elastic workpiece recovery.

Since the operator provided initial setup conditions or the initial roll opening settings provided by an associated computer control system operative with model equation information to calculate the setup screwdown schedules for the rolling mill, can be in error and since in any event certain mill parameters affect the stand loaded roll opening during rolling and after setup conditions have been established, a stand automatic gauge control system must be employed if it is necessary that the stand delivery gauge be closely controlled. Thus, at the present state of the rolling mill art and particularly the steel rolling mill art, a stand gauge control system is normally used for a reversing mill stand and for predetermined stands in tandem rolling mills.

More particularly, the well known gaugemeter or roll force system has been widely used to produce stand gauge control in metal rolling mills and particularly in tandem hot steel strip rolling mills and reversing plate mills where experience has demonstrated that roll force control is particularly effective. Earlier publications and patents such as an article entitled Installation and Operating Experience with Computer and Programmed Mill Controls by M. D. McMahon and M. A. Davis in the 1963 Iron and Steel Engineer Year Book at pages 726 to 733, an article entitled Automatic Gage Control for Modern l-Iot Strip Mills by J. W. Wallace in the December 1967 Iron and Steel Engineer at pages 75 to 86, U.S. Pat. No. 3,561,237 issued Feb. 9, 1971 to Eggers et al, and US. Pat. No. 2,726,541, issued Dec. 13, 1955 to R. B. Sims describe the theory upon which operation of the roll force and related gauge control systems is based. Attention is also called to US. Pat. No. 3,568,637 issued Mar. 9, 1971, US. Pat. Nos. 3,574,279 and 3,574,280 issued Apr. 13, 1971, and U.S. Pat. No. 3,600,920 issued Aug. 24, 1971 to A. W. SMith, which relate to roll force automatic gauge control systems. In referencing prior art publications or patents as background herein, no representation is made that the cited subject matter is the best prior art.

Briefly, the roll force gauge control system uses Hooks law in controlling the screwdown position at a rolling stand, i.e., the loaded roll opening under workpiece rolling conditions equals the unloaded roll opening or screwdown position plus the mill spring stretch caused by the separating force applied to the rolls by the workpiece. To embody this rolling principle in the roll force gauge control system, a load cell or other force detector measures the roll separating force at each controlled roll stand and the screwdown position is controlled to balance roll force changes from a reference value and thereby hold the loaded roll opening at a substantially constant value. The following well known formula expresses the basic roll force gauge control relationship:

where: g h loaded roll opening (workpiece delivery gauge'or thickness) S0 unloaded roll opening (screwdown position) K mill spring constant F roll separating force. Typically, the roll force gauge control system is an analog arrangement including analog comparison and amplification circuitry which responds to roll force and screwdown position signals to control the screwdown position and hold-the following equality:

AS AF-K (2) where: AF measured change in roll force from an initial force AS controlled change in screwdown position from an initial screwdown position. After the unloaded roll opening setup and the stand speed setup are determined by the mill operator for a particular workpiece pass or series of passes, the rolling operation is begun and the screwdowns are controlled to regulate the workpiece delivery gauge from thereversing mill stand or from each roll force controlled tandem mill stand. By satisfying Equation (2), and the assumptions implicit in Equation (1), the loaded roll opening h in Equation (1) is maintained constant or nearly constant.

As the head end of the workpiece strip enters each roll stand of the mill, the lock-on screwdown position and the lock-on roll separating force are measured to establish what strip gauge should be maintained out of that roll stand. As the strip rolling operation proceeds, the roll stand separating force and the roll stand screwdown position values are monitored and any undesired change in roll separating force is detected and compensated for by a corresponding correction change in screwdown position. The lock-on gauge LOG is equal to the lock-on screwdown LOSD plus the lock-on force LOF multiplied by the mill stand spring modulus K. The workpiece strip delivery gauge G leaving the roll stand at any time during the rolling operation is in accordance with above equation (I) and is equal to the unloaded screwdown position SD plus the rollseparating force F multiplied by the mill spring modulus K.

The gauge error is derived by subtracting the lock-on guage from the delivery gauge. The following Equations 3, 4 and 5 set forth these relationships.

LOG LOSD K*LOF G SD K*F G LOG GAUGE ERROR [SD LOSD] One mill condition which can cause steady state gague error is an incorrect operator setup. Thus, the screwdown position and the stand speed setup at a particular stand results in a head end stand delivery gauge which may or may not equal the head end gauge desired from the setup values. If the roll force control uses a head end lock on roll force reference, the stand is roll force controlled to continue rolling the actual head end gauge unless the screwdowns are externally offset to produce the correct steady state gauge.

The initial screwdown position calibration is a direct electromechanical measurement technique made at the beginning of work roll life and if desired new initial calibrations are made at various subsequent time points in the work roll life. In any case, the predetermined initial screwdown calibration is subject to change during mill operation and any such change-requires screwdown offset for correction of the roll force control operation. Typically, calibration drift is caused by changes in roll stand heating, stand speed (bearing oil film thickness), roll wear, differential leveling operation of the screwdowns for shape control and possibly by changes in other mill conditions.

When the initial screwdown calibration does drift, changes occur in the screwdown position at which roll facing occurs thereby making the unloaded roll opening correspondence with screwdown position differ from the initial correspondence by the amount of the calibration drift. As a result, the actual loaded roll opening, i.e. the actual gauge, differs from the expected value calculated with the use of an unloaded roll opening which is based on the erroneous calibration. The difference represents a gauge error condition which is correctable by a screwdown offset or, more specifically, a screwdown recalibration. If the mill spring constant'changes, the actual loaded roll opening differs from the expected value calculated with the use of a mill stretch which is based on the erroneous mill spring constant, and the resultant gauge error condition is normally similarly correctable by a screwdown offset.

To provide steady state gauge error correction, the well known monitor gauge control system is usually employed to produce screwdown offset for the roll force controls. In the monitor system, an X-ray or other radiation gauge is placed at one or more predetermined process points and usually at least at a process point following the delivery end of the mill in order to sense actual delivery gauge after a workpiece transport delay from the point in time at which the actual delivery gauge is produced at the preceding stand or stands. The monitor system compares the actual delivery gauge with the desired delivery gauge and develops an analog feedback control signal to adjust the operation of the reversing mill roll force gauge control system or one or more predetermined tandem mill stand roll force gauge control systems to supply desired steady state mill delivery gauge. In this manner, the conventional monitor system provides for transport delayed correction of steady state gauge errors which are caused or which are tending to be caused by a single mill variable or by a combination of mill variables.

In operator controlled mills, some steady state gauge correcting load can eventually be taken off the monitor system by screwdown recalibration, and the like, between workpiece passes if steady state gauge error tends to exist along the entire workpiece and persists from workpiece to workpiece. In this manner, some re.- duction is achieved in the length of off gauge workpiece material otherwise associated with monitor transport delay. Similarly, corrective monitor system operation caused by head end gauge errors can be reduced by changes in the operator or associated computer con trol system provided setup from workpiece to workpiece.

A background teaching of stored program digital computer system operation can be found in a book entitled Electronic Digital Systems by R. K. Richards and published in 1966 by John Wiley and Sons.

A more detailed description of computer programming techniques in relation to the control of metal rolling mills can be found in an article in the Iron and Steel Engineer Yearbook for i966 at pages 328 through 334 entitled Computer Program Organization for an Automatically Controlled Rolling Mill by John S. Deliyannides and A. H. Green, and in another article in the Westinghouse Engineer for January 1965 at pages 13 through 19 and entitled Programming for Process Control by P. E. Lego.

SUMMARY OF THE INVENTION In accordance with the broad principles ofthe present invention, a system and method for controlling a gauge in a metal rolling mill employs means for detecting at least one error condition representing gauge error and means for controlling screwdown position at each of one or more predetermined rolling stands of the mill. Means are also provided for determining the total amount of screwdown movement (position change) required to correct the error condition at predetermined mill spring constant and workpiece plasticity values. In roll force gauge control, the roll force is detected and the determined error conditions is a gauge error.

There is calculated a correction in relation to a gauge error or X-ray gauge deviation measured after the last roll stand which correction is for selected roll stands previous to the last roll stand. This calculated correction is determined from the rolling of an initial workpiece strip for adjusting the roll opening settings of the selected roll stands before the subsequent rolling of another workpiece strip similar to that initial workpiece strip. A selected number of previous roll stands are corrected while the last stand is roll force gauge controlled. This correction is determined by a mass flow relationship with the speed of the last stand and the speed of the corrected stand, the measured gauge deviation and a predetermined weighting factor.

A digital computer system is preferably employed to make the error correction screwdown movement determinations as well as to perform other mill control functions. The computer employs a programming system including an automatic roll force gauge control program or AGC program which is executed at predetermined intervals to calculate the desired screwdown movement required at each roll force gauge controlled stand for gauge error correction including that stemming from roll force error detection at that stand. Screwdown movement for correcting roll force error is made on the basis of calculations which use selected workpiece plasticity and mill spring constant values stored in data tables in the computer system memory or otherwise determined by the computer system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of a tandem hot steel strip rolling mill and a digital computer system au tomatic gauge control therefor arranged for operation in accordance with the principles of the invention;

FIG. 2 illustrates a mill spring curve and a workpiece reduction curve for a given rolling mill stand and the manner in which roll force screwdown correction is determined in relation to a change in the stand load force;

FIG. 3 illustrates a mill spring cruve and a workpiece reduction curve for a given rolling mill stand and the manner in which roll force screwdown correction is determined in relation to a change in both the stand load force and the stand screwdown position setting;

FIG. 4 shows an illustrative mill stand deflection curve and product deformation curve to show the required screw movement to correct a determined gauge error;

FIG. 5 shows a graphic representation of typical PW plasticity values, such as could be included in the use table as loaded into the computer control system memory or such as could be included in the adaptive table.

FIGS. 6A, 6B and 6C show an illustrative logic flow chart of an AGC program operative with a tandem hot strip rolling mill;

FIGS. 7A and 7B show an illustrative logic flow chart of an X-ray routine program operative with the AGC program shown in FIGS. 6A, 6B and 6C;

FIG. 8 shows an illustrative logic flow chart of a feed forward routine program operative with the above AGC program shown in FIGS. 6A, 6B and 6C;

FIG. 9 shows an illustrative logic flow chart of a feedback routine program operative with the AGC program shown in FIGS. 6A, 6B and 6C;

FIG. 10 shows an illustrative logic flow chart of the adaptive routine program operative with the X-ray routine program shown in FIGS. 7A and 7B; and

FIG. 11 shows an illustrative logic flow chart of the head end routine program operative with the X-ray routine program shown in FIGS. 7A and 78.

GENERAL DESCRIPTION OF THE AUTOMATIC GAUGE CONTROL SYSTEM AND ITS OPERATION There is shown in FIG. 1 a tandem hot strip steel finishing mill ll operated with improved gauge control performance by a process control system 13 in accordance with the principles of the invention. Generally, however, the invention is applicable to various types of mills in which roll force gauge control is employed. Thus, the invention can be suitably adapted for application in hot steel plate reversing and other rolling mills.

The tandem mill 11 includes a series of reduction rolling stands with only two of the stands S1 and S6 shown. A workpiece 15 enters the mill 11 at the entry end in the form of a bar and it is elongated as it is transported through the successive stands to the delivery end of the mill where it is coiled as a strip on a downcoiler 17. The entry bar would be of known steel grade and it typically would have a thickness of about 1 inch and a width within some limited range such as 20 inches to inches. The delivered strip would usually have approximately the same width and a thickness based upon the production order for which it is intended.

In the reduction rolling process, the successive stands operate at successively higher speeds to maintain proper workpiece mass flow. Each stand produces a predetermined reduction or draft such that the total mill draft reduces the entry bar to strip with the desired gauge or thickness.

Each stand is conventionally provided with a pair of backup rolls l9 and 21 and a pair of work rolls 23 and 25 between which the workpiece 15 is passed. A large DC drive motor 27 is controllably energized at each stand to drive the corresponding work rolls at a controlled speed.

As' previously described, the sum of the unloaded work roll opening and the mill stretch substantially defines the workpiece gauge delivered from any particular stand in accordance with Hookes law. To vary the unloaded work roll opening at each stand, a pair of screwdown motors 29 (only one shown at each stand) position respective screwdowns 31 (only one shown at each stand) which clamp against opposite ends of the backup rolls and thereby apply pressure to the work rolls. Normally, the two screwdowns 31 at a particular stand would be in identical positions, but they can be located in different positions for strip guidance during threading, for flatness or other strip shape control purposes or possibly for other purposes.

A conventional screwdown position detector or encoder provides an electrical representation of screwdown position at each srand. To provide an absolute correspondence between the screwdown position and the unloaded roll opening between the associated work rolls, a screwdown position detection system which includes the screwdown position detector 33 can be calibrated from time to time in the manner previously described.

Roll force detection is provided at each of predetermined stands by a conventional load cell 35 which generates an electrical analog signal. At the very least, each roll force controlled stand is provided with a load cell 35 and in many cases stands without roll force gauge control would also be equipped with load cells. The number of stands to which roll force gauge control is applied is predetermined during the mill design in accordance with cost-performance standards, and increasingly there is a tendency to apply roll force gauge control to all of the stands in a tandem hot strip steel mill. In the present case, a roll force gauge control system is assumed to be employed at each of the stands.

Conventional motorized sideguards 37 are located at predetermined points along the mill length. The sideguards 37 are operated during mill setup on the basis of the widths of the upcoming workpiece 15 thereby defining the sides of the workpiece travel path for guidance purposes.

The process control system 13 provides automatic control for the operation of the tandem mill 11 as well as may be desired for associated production processes (not indicated) such as the operation of a roughing mill. Preferably, the process control system 13 comprises a programmed process control digital computer system 39 which is interfaced with the varoius mill sensors and the various mill control devices to provide control over many of the various functions involved in operating the tandem mill 11. According to user preference, the control system 13 can also include conventional manual and/or automatic analog controls for backup operation in performing preselected mill functions.

On the basis of these considerations, the digital computer system 39 includes a finishing mill on-line roll force gauge control computer system, such as a Prodac 2000 (P2000) sold by Westinghouse Electric Corporation. A descriptive book entitled Prodac 2000 Computer Systems Reference Manual has been published in 1970 by Westinghouse Electric Corporation and made available for the purpose of describing in greater detail this computer system and its operation.

The computer processor is associated with well known predetermined input systems typically including a conventional contact closure input system which scans contact or other signals representing the status of various process conditions, a conventional analog input system which scans and converts process analog signals, and operator controlled and other information input devices and systems 41 such as paper tape teletypewriter and dial input systems. It is noted that the information input devices 41 are generally indicated by a single block in FIG. 1 although different input devices can and typically would be associated with the computer system 39. Various kinds of information are entered into the computer system 39 through the input devices 41 including, for example, desired strip delivery gauge and temperature, strip entry gauge and width and temperature (by entry detectors if desired), grade of steel being rolled, plasticity tables, hardware oriented programs and control programs for the programming system, and so forth.

The contact closure input systems and the analog input systems interface the computer system 39 with the processes through the medium of measured or detected variables. The present invention is largely involved in the functioning of the automatic gauge control computer system, hereinafter referred to as the AGC computer. In one typical invention application, various mill signals are applied to the AGC computer input systems. These mill signals include the following:

1. A roll force signal from the load cell 35 at each stand proportional to stand roll separating force for use in roll force gauge control.

2. Screwdown position signals generated by the respective detectors 33 at the stands for use in roll force gauge control.

3 Screwdown motor speed signals generated by respective tachometers 49 at the stands for use in programmed regulation.

4 Stand speed signals generated by respective tachomctcrs 43, with the speed signal used for calculation of acceleration compensation and for calculation of time delays in monitor operation.

5 A gauge deviation signal from an X-ray gauge 47 at the delivery end of the mill for programmed monitor gauge control through the roll force control.

6 An entry temperature signal from a mill entry temperature detector or pyrometer 45; the mill entry temperature for the head end of each workpiece 15 is stored.

7 Width signals supplied by sideguard follow potentiometers for mill spring constant calculations, etc.

It is noted at this point in the description, that the measured head end roll force is stored and used as a reference for roll force gauge control functioning at the respective stands if the AGC computer is in the lock-on mode of roll force operation.

A contact closure output system would normally be associated with the digital computer system 39. In the operation of the AGC contact closure output system, various control devices are operated in response to control actions calculated or determined by execution of control programs in the AGC computer.

To effect determined control actions, controlled devices are operated directly by means of output system contact closures or by means of analog signals derived from output system contact closures through a digital to analog converter. The principal control action outputs from the AGC computer contact closure output system include screwdown positioning commands which are applied to respective screwdown positioning controls 55 in operating the screwdown motors 29 for screw movement, and speed control signals which are applied to the respective speed and tension control system 53 to cause a change in drive speed to compensate the force on the strip for a change in thickness being made by a screwdown movement.

Display and printout systems 51 such as numeral display, tape punch, and teletypewriter systems are also associated with the outputs of the digital computer system 39 in order to keep the mil-l operator generally informed about the mill operation and in order to signal the operator regarding an event or alarm condition which may require some action on his part. The printout systems are also used to log mill data according to computer log program direction.

Generally, the AGC computer uses Hookes law to determine the total amount of screwdown movement required at each roll force controlled stand at the calculating point in time for roll force and gauge error correction, i.e., for loaded roll opening and stand delivery gauge correction to the desired value. The calculation defines the total change in the unloaded roll opening required to offset a new mill stretch value or other roll force and gauge error causing condition. The predicted corrective screwdown position change value is employed in a screwdown position control program in the AGC computer to define the screwdown motor position-time profile to be followed in making the corrective screwdown movement.

During rolling operation, the on line gauge control system operates the stands to produce strip product having desired gauge and proper shape, i.e., flat with slight crown. On line gauge control is produced by the roll force gauge control loops at the stands and the previously noted screwdown monitor gauge control system.

In the monitor system, the X-ray gauge 47 produces the previously indicated X-ray deviation signal which indicates the difference between actual strip delivery thickness and desired or target strip delivery thickness. In other cases, it may be desirable to employ an absolute thickness measurement X-ray gauge signal to form a basis for monitor control actions or, more generally, for screwdown offset control actions.

To effect on line gauge control in the closed loops, the programmed AGC computer system operates on the screwdown position detector and load cell signals from each stand as well as the X-ray gauge deviation signal to determine the control actions required for producing desired strip delivery gauge. Screwdown motor speed is in this instance also applied to the computer system 39 in order to provide for programmed screwdown positioning control. In effecting control operations, the AGC computer employs an AGC. programming system which forms a part of the total programming system for the computer system 39. The AGC programming system includes programs oriented to controlling the AGC computer system hardware and programs oriented to developing the control actions.

In FIG. 2, curves are shown to illustrate the application of Hookes law to a rolling mill stand and to illustrate the unique basis upon which the process control system 13 and in particular the on line AGC computergauge control system provides improved gauge control speed, accuracy and stability and other operating benefits. A mill spring curve defines the separation between a pair of mill stand work rolls as a function of separating force and as a function of screwdown position. The slope of the mill spring curve is the well known mill spring constant K which is subject to variation as previously described. When a correct screwdown calibration is known and the screwdowns are positioned such that the empty work rolls are just facing, the unloaded screwdown zero position is defined. The zero screwdown location mill spring curve is indicated by the reference character 61.

At the correct calibration condition, the indicated theoretical face intersect represents theoretical roll facing and it is for this theoretical condition that the screwdown position is assigned to a zero value. Under the correct calibration condition, roll facing actually occurs when the screwdown position is at a slightly negative value because of the nonlinearity of the lower part of the mill spring curve. A definition of the screwdown calibration as being correct for the indicated theoretical condition is, however, convenient and appropriate for mill operation.

When the screwdowns are opened (positive movement) the unloaded roll opening increases as reflected by a change to the right in the graphical location of the mill spring curve as indicated generally by crve 67 such that the theoretical spring curve intersect equals the new unloaded roll opening. With screwdown closing, the mill spring curve is shifted to the left in a similar manner.

At any particular screwdown position and with correct screwdown calibration, the stand workpiece delivery gauge equals the unloaded roll opening as defined by the screwdown position S plus the mill stretch caused by the workpiece. If the screwdown calibration is incorrect, i.e., if the number assigned to the theoretical roll facing screwdown position is something other than zero because of roll crown wear or other causes,

the stand workpiece delivery gauge equals the unloaded roll opening plus the mill stretch plus or minus the calibration drift.

The amount of mill stretch depends on the characteristic reduction curve for the workpiece. As shown in FIG. 2, a reduction curve 65, for a strip of predetermined width represents the amount of force required to reduce the workpiece from a stand entry thickness (height) of H The workpiece plasticity P is the slope of the curve 65, and in this case the curve is shown as being linear although a small-amount of nonlinearity would normally exist.

Desired workpiece gauge H is the initial condition IC produced in this case since the amount of force required to reduce the workpiece from H to H is equal to the amount of roll separating force required to stretch the rolls to a loaded roll opening H i.e., the intersection of the mill spring curve at an initial screwdown opening S indicated by mill spring curve 67 and the workpiece reduction curve 65 lies at the desired gauge value.

As shown in FIG. 2, if the stand delivery gauge increases by a gauge error amount GE to B during a workpiece pass to produce a present condition PC, in this instance because the workpiece plasticity decreases and because the workpiece entry thickness increases to I-l as represented by the reduction curve 69, the stand screwdowns must be closed to a value which causes a future correct gauge condition FC. At the condition PC, the intersection of the mill spring curve and the new reduction curve 69 lies at the desired gauge I-I as provided by a spring curve location indicated by the reference character 63. In other words, corrective screwdown closing causes the unloaded screw opening to be reduced by an amount A S to a new value which adds with the new mill stretch to equal the desired gauge H As shown in FIG. 3, after the stand screwdowns are moved from the initial position S to another position S,,, the force error FE and the related gauge error GE must not only take into consideration the change in force from the initial value F but also the change in screwdown from the initial position S The correction required in the screwdown position is A S to produce the desired gauge H and the new screwdown position S is o o A SRF where: S, is the present unloaded screwdown position A S is the required correction in the screwdown position.

In accordance with the present invention, A S is calculated to enable roll force gauge control operation in accordance with the following programmed algorithm:

A S [K/P +1] GE A S required screwdown correction where:

GE gauge error K mill spring constant (in/10 1b) P workpiece plasticity (in/10 1b) Equation 7 is derived with reference to FIG. 3 as follows:

GE FEK gauge error A F GE/P expected change in roll force resulting from corrective screwdown movement. 9 AS =AF-K+AF-P=AF*[K+P] 10 AS =GE/P*[K+P]=GE*[K/P+l] (ll) In order to calculate the predicted amount of screwdown movement required to correct a gauge error, the gauge error GE is calculated as follows:

GE x FR] o S10) In providing for the gauge error calculations Equation 12 defines the difference between the present roll force F and the reference roll force F (either lock on or absolute as predetermined) in relation to the stand mill spring constant and subtracts from that difference the amount of change in roll force caused by screwdown movement made to correct previous roll force error. For the condition PC shown in FIG. 2, S S in Equation l2, but in general 8,, would typically have some value other t hari S as shown in FIG. 3.

Corrective screwdown movement in the predicted amount produces further roll force change and FE becomes zero if the system behavior corresponds to predictions and if no new roll force error develops during the period of correction. If the system does not behave as predicted, FE does not become zero and in efiect a new roll force error PE is generated to the extent that the executed screwdown movement in the predicted amount fails to correct the stand delivery gauge.

It is also noted at this point in the description that the screwdown reference S used as a base for determining the gauge error GE in Equation 12 is updated as follows:

10 SM an where: S Screwdown offset produced by conventional X-ray monitor operation S screwdown offset produced for roll force error anticipated by feedforward action. These quantities are considered more fully in the disclosure of the above referenced US. Pat. No. 3,561,237. By way of explanation, the screwdown reference S can be up-dated in accordance with Equation 13 as changes occur in S and 8,, in order to prevent the stand roll force gauge control system from responding to roll force changes caused solely by screwdown movement required by external screwdown offset system control for screwdown calibration, head end gauge error correction in the lock on mode of operation, anticipatory mill speed change compensation, anticipatory roll force error compensation or other gauge error correcting purposes. If conventional X-ray monitor is not employed in the system 13, the corresponding term 8,, can be omitted from Equation 13.

Generally the operative value of each stand spring constant K is relatively accurately known. It is first determined by the conventional work roll screwdown test, and it is recalculated prior to each workpiece pass on the basis of the workpiece width and the backup roll diameter. Each resultant spring curve 61 is stored for on line gauge control use.

The form in which the spring constant K is stored can vary. In the present case, the slope of the linear part of the spring curve is stored as a single value. The nonlinear part of the spring curve is estimated by three straight lines of increasing slope with the respective slopes stored as three separate spring constant values which are corresponding force range. As future mill data returns from computer data logging demonstrate presently unknown relationships which may define on line variations of the mill spring constant as a function of certain mill variables, provision can be made for programming on line calculations of the mill spring constant in accordance with such relationships under dynamic mill operating conditions.

The operative value of the workpiece plasticity P at each stand is also relatively accurately determined, in the present case, P tables are stored in the computer system 39 to identify the various values of P which apply to the various mill stands for various grade class and gauge class workpiecesunder various operating conditions and at various operating times during the rolling of the strip 14. The plasticity values are stored in the table as a plasticity for a product with a width of unity, typically in inches/l0 pounds/in wide. The values in the table, PW, are divided by the width of the product being rolled to obtain the appropriate value.

Hot strip mill gauge control using programmed digital computer evaluation of the roll force feedback information involves the combination of a number of process control operations. Roll force, screw position, and mill spring information is used to evaluate the gauge of the strip as it is worked in each stand, and an X-ray gauge is used on the strip as it passes out of the last stand to evaluate the absolute strip gauge produced by the rolling mill.

A multi-stand and continuous hot strip mill requires a gauge control system to maintain uniform gauge. Typically a hot strip mill will roll a single strip simultaneously in all of its stands. Therefore the gauge control system used with the mill should be able to determine gauge errors leaving each of the stands as quickly as possible, and it should be able to make corrections to those gauge errors in as many stands as may be necessary.

There are two gauge error detection systems used for this purpose to consider: (1) X-ray and (2) roll force. X-ray gauge measuring devices should be placed after each roll stand; they are accurate, but they are expensive, difficult to maintain, and can only detect errors after the workpiece strip has passed the provided distance between the associated roll stand and the subsequent gauge measuring X-ray device. On the other hand, the roll force gauge error detection system is much less expensive, and can be more easily implemented in all roll stands; it detects errors in the workpiece strip gauge as the strip is still passing between the rolls, thereby allowing more immediate evaluation of required corrections to the roll stand screwdown position setting. Unfortunately, the roll force system provides only a relative evaluation of the strip gauge, since it measures how much the strip gauge has deviated from the gauge of the head end portion of the strip.

A practical combination of the above two gauge error detection systems is to use roll force feedback information to calculate fast desired corrections to errors in strip gauge, and to use one X-ray gauge measuring device after the last roll stand to evaluate the absolute gauge of the strip coming out of the last stand. The fast corrections are calculated from the roll force feedback information, combined with the detected stand screwdown position and the predetermined modulus of elasticity of the rolling stand. The slower X-ray device gauge error evaluation is used to calculate simultaneous monitor corrections to several stands so that the absolute value of the strip gauge leaving the rolling mill may be brought to the desired value. The output of each of these systems is a screwdown correction gauge in the respective positions of the screwdowns of each of the stands.

FIG. 2 shows the linear approximations of the mill deflection curve 67 and the product deformation curve 65 for a typical rolling mill stand operation. The unloaded roll opening S' sometimes called the screwdown because of the screw and nut system used for adjusting the roll opening, is the strip gauge that would be delivered if there were no roll separating force. As the stand roll force increases with a constant roll opening, the delivery strip gauge increases, since the mill stand stretches or deflects. This is shown by the line 67 with slope K. The product deformation characteristic is represented by the line 65 with slope P. If there were no force exerted by the roll stand on the product being rolled, the strip gauge would not be reduced and the delivery strip gauge would be equal to the entry strip gauge. If the roll force is caused to increase, the product is plastically deformed and the delivery strip gauge decreases. The slope of the mill characteristic line 67 is called the mill spring modulus K, and the slope of the product deformation characteristic line 65 is called the product plasticity P.

The delivery strip gauge is determined by the equilibrium point IC where the force exerted by the mill stand is equal to the force required to deform the product. Changes in workpiece strip entry gauge and/or changes in product hardness result in a change in stand roll force and delivery strip gauge. The gauge error correcting control system must move the screwdown to correct for these resulting error changes in strip gauge.

The main advantage of using the roll force gauge control system is the ability to detect error changes in strip gauge the instant they take place as the product is being rolled in the roll stand. A shift in strip delivery gauge or thickness can be caused by a change in entry thickness, or a change in hardness as usually caused by a change in temperature. This change in delivery gauge can be immediately detected by feedback information monitoring of the roll separating force on the roll stand.

FIG. 3 illustrates the operation of a gauge error detection system, with the mill spring line 67 and product plasticity line 65 representing the initial lock-on condition of operation and the mill spring line 63 and plasticity line 69 representing the future condition. As compared to the original lock-n conditions, the screwdown system has moved in the closing direction and the roll separating force has increased because of a harder cold portion of the workpiece strip passing through the roll stand. v

The required gauge error correction in screwdown position is not only dependent on the strip gauge error but also on the stand mill spring modulus and the product plasticity values. In FIG. 3 there is illustrated how the gauge error GE is removed by a screwdown correction A S The screwdown correction A S is larger than and approximately twice the size of the gauge error GE, since this correction operation will actually result in an increase in stand roll force because of the greater reduction taken. Relatively soft workpiece strip products require a screw correction A S very nearly the same as the gauge error GE but relatively hard products require a larger correction compared to the gauge error. The necessary screwdown movement A S to correct a determined gauge error is determined as follows in relation to FIG. 4. The screwdown correction A S can be determined by the realtionship:

where: I X is the amount of roll opening change and hence strip delivery gauge change due to the stretch of the roll stand GE is the gauge error.

The roll stand stretch X can be determined by the relationship:

where: A F is shown in FIG. 4 and is the change in roll force when the gauge error GE is corrected.

From the illustration shown in FIG. 4, it is also seen that GE=P* AF AF=GE/P now combining Equation 9 with Equation 15 will give and combining Equation 17 with Equation 14 will give A S K* (GE/P) GE A SR: GE*

The screwdown correction A S is shown in FIG. 4 in relation to the gauge error GE, the desired gauge H D and the present gauge H FIG. 5 shows a graphic representation of typical PW values stored for a seven stand tandem hot strip mill. The six sets of values cover the range of thicknesses rolled from 0.050 inches to- 0.250 inches. This could be an illustration of a use table as loaded by the operator into the computer storage memory or it could be an illustration of a learned table provided by the adaptive program.

The main roll force AGC program, as illustrated by the flowchart shown in FIG. 6, and the associated X-ray monitor program, as illustrated by the flowchart shown in FIG. 7, occupy the same task priority level, with both programs being initiated by the well known analog input signal scan operation. The roll force AGC program operationally maintains a constant workpiece gauge based on the initial mill setup parameters provided by the operator, while the X-ray program backs up the roll force AGC control by monitoring the final product gauge and making desired adaptive corrections as needed.

When a workpiece-strip is being rolled, the roll force AGC program corrects deviations from the initial or head end exit gauge of each stand, by adjusting the screw opening of the stand. A deviation in the exit gauge of a stand from its initial or head end lock-on value is reflected in a sensed change in the roll force of the stand from its initial value. From the exit gauge error when scanned and determined every 2/ 10 second, a gauge error correction to the screw opening is determined and made by adjusting the screwdown position of the respective roll stand.

The main AGC program also includes a subroutine for making corrections to a stands entry gauge by feed- 7 ing back determined gauge error under specified conditions to the screw opening of the previous stand, this feedback program subroutine is illustrated by the flowstand can help with this desired gauge error correction needed in relation to the operation of that given stand.

The flow charts shown in FIGS. 6, 7 and 8 are written in an effort to be substantially self-explanatory to persons skilled in this particular art, with the functions to be performed at each step of the flow charts being set forth accordingly.

in FIGS. 6A, 6B and 6C there is shown a flow chart to illustrate a preferred embodiment of a suitable AGC program operative with a tandem hot strip rolling mill. At step 10 a determination is made to see if the automatic gauge control or AGC program has been selected and desired by the operator to be functional. The AGC program is run shortly after the head end of a workpiece strip has entered that standsof the rolling mill, and for each roll stand the initial lock-on roll force,

lock-on speed and lock-on screwdown position setting is measured and saved in memory storage. At step 12 a determination is made to see if this particular scan is not an X-ran scan; for a typical rolling mill installation, there may be seven roll stands to be scanned plus a scan of the X-ray device located after the last roll stand in relation to the provision of analog input signals to be scanned by the digital computer system. This step 12 procedure relates to the organization of the analog signal inputs; if this is an X-ray input signal scan, the program goes to the X-ray subroutine at step 14, and if this is not an X-ray scan and instead this is an AGC program run in relation to one of the drive stands the program goes to step 16 to determine if roll force has been selected by the operator for this stand, i.e., stand N, where N can be each one of the roll stands in sequence. Step 18 is provided to see if automatic scan has been selected by the operator for this stand. Each of steps 16 and 18 must be satisfied, or the program goes to step 20 for reset of a head end software flag and to step 22 for turn-off of the AGC light for that stand on the operators control panel. The operator has a roll force select switchby which he initiates the roll force AGC program operation. At step 24 a determination is made to see if the screwdown positioning mechanisms for this stand have been calibrated. lf they have, at step 26 the program checks to see that the workpiece strip is in this stand, and at step 28 to see if this is the first scan made on this particular strip. Each of these conditions has to be satisfied for the AGC program to run through for this stand as desired. For the first scan on this strip, the measured lock on force for this stand, the lock on speed and the lock on screwdown position is saved at step 30, since these parameter values will be later needed for control purposes.

At step 32, the measured lock on roll force for this stand is used with a predetermined look-up table of mill spring modulus values provided in'storage, in relation to the well known nonlinear mill spring characteristic for a typical roll stand, to determine the value of the mill spring modulus K to use for subsequent calculations in relation to the lock on operation of this stand. it should be noted that the upper portion of the mill spring characteristic curve is well known by persons skilled in this art to be substantially linear as shown in FIGS. 2 and 3 above an initial lower portion for the typical roll stand and in accordance with the disclosure of U.S. Pat. No. 2,726,541 of R. B. Sims. At step 34 the stand mill spring modulus K is corrected in relation to the known width of the workpiece strip. At step 36 the exit gauge class for this stand is determined, such that at step 38 an adaptive learned look-up table operation will provide the average plasticity PW for this gauge class. At step 40 the plasticity constant P is calculated in relation to this determined average value PW divided by the known width of the workpiece strip. At step 42 the desired hardness correction in relation to known grade of the strip being rolled is determined by a predetermined table look-up operation with operator provided values of same. At step 44 the plasticity P for this stand is corrected for hardness.

At step 46 the stand screw position is read. It should be noted that if the present scan was not the first scan on this particular strip at step 28, the program then went to step 46, through NOT HEAD which is referenced in the program listing operative with the digital computer. At step 48, if the stand load cell is ON, the strip gauge error is calculated at step S0 in accordance with above Equation 13. At step 52 the desired screw position correction A Sap is calculated in accordance with above equation 11. At step 54 a check is made to see if the desired screw correction A S is greater than an operator determined minimum response deadband value; if this screw correction is not greater than this deadband value, at step 56 it is made zero, and is this screw correction is greater than the deadband, at step 58 the running average gauge error is calculated. At step 60 if the screw correction is not within the operator defined maximum correction limits, the program advances to step 62 where the screw condition is set to equal that maximum correction limit and the feed forward subroutine is entered by step 64 to feed forward some of the needed gauge error correction from the stand to the later stands of the rolling mill. At step 66 a check is made to see if a feed forward correction from a previous stand is approaching this stand, such as would happen in relation to a skidmark. if it is, a determination is made at step 68 if the skidmark is about to 

1. A gauge control system for a rolling mill having at least a first roll stand and a second roll stand operative with initial roll opening settings to reduce the gauge of a plurality of similar workpieces passed through said rolling mill, said system comprising: means for measuring the gauge deviation of a first of said workpieces leaving said second roll stand, means for determining a correction for adjusting the roll opening setting of at least said first roll stand for the passage of at least a second workpiece in accordance with a predetermined mass flow relationship between said gauge deviation of said first workpiece, the operating speed of said first roll stand and the operating speed of said second roll stand, and, means for controlling the roll opening of said first roll stand for the passage of at least said second workpiece in accordance with said gauge correction.
 2. The gauge control system for a rolling mill of claim 1, including means for controlling the roll opening of said second roll stand in accordance with the measured roll force of that second roll stand and the predetermined mill spring characteristic of that second roll stand.
 3. The gauge control system of claim 1, with said rolling mill having a plurality of roll stands and said second roll stand being the last roll stand of the rolling mill, said system including means for controlling the roll opening of a third roll stand positioned between said first roll stand and said last roll stand during the passage of at least said second workpiece in accordance with said correction, with said relationship including a weighting factor having a first value when said correction is determined for adjusting the roll opening of said first roll stand and having a second and different value when said correction is determined for adjusting the roll opening of said third roll stand.
 4. The gauge control system for a rolling mill of claim 1, with said rolling mill having a plurality of roll stands operative to reduce the gauge of said plurality of workpieces passed through said rolling mill, with said correction being determined by the relationship CORRECTION (RPMLS) GAUGE DEVIATION/(RPMTS) WEIGHTING FACTOR, where the RPMTS is the operating speed of said first roll stand, where RPMLS is the operating speed of said second roll stand and the weighting factor is determined in relation to a desired portion of said determined correction that is to be applied to said first roll stand.
 5. The gauge control system for a rolling mill of claim 1, with said second roll stand being controlled by a roll force gauge control system responsive to a roll force determined workpiece gauge leaving said second roll stand.
 6. A method of controlling the workpiece gauge leaving a rolling mill having at least a first roll stand operative with initial roll opening settings to reduce the gauge of a plurality of similar workpieces passed through said rolling mill and including a device for measuring the gauge deviation of a first workpiece leaving said rolling mill, the steps of said method comprising: determining a roll opening correction for application to said first roll stand during the passage of at least a second workpiece in accordance with a predetermined mass flow relationship with said gauge deviation of said first workpiece and the workpiece movement from said rolling mill as compared to the workpiece movement from said first roll stand, and controlling the operation of said first roll stand in accordance with a roll opening setting modified by said correction.
 7. The method of controlling the workpiece gauge leaving a rolling mill of claim 6, with said rolling mill having a plurality of roll stands operative to reduce the gauge of said plurality of workpieces passed through said rolling mill, with said method including: determining the roll opening correction for application to each of selected roll stands during the passage of at least said second workpiece in accordance with said predetermined relationship and a predetermined weighting factor for each of said selected roll stands such that a decreasing portion of said correction is applied to each Selected roll stand in relation to the distance of the selected roll stand ahead of the last roll stand of the rolling mill.
 8. The method of controlling the workpiece gauge leaving a rolling mill of claim 6, with said rolling mill having a plurality of roll stands and with said plurality of workpieces being similar in at least one of gauge classification and grade classification, said method including: determining a roll opening correction for application to each of selected roll stands during the passage of at least said second workpiece such that the roll opening setting of an earlier selected roll stand is corrected less than the roll opening setting of a later selected roll stand.
 9. The method of controlling the workpiece gauge leaving a rolling mill of claim 6, with said rolling mill having a plurality of roll stands operative to reduce the gauge of said plurality of workpieces passed through said rolling mill, the steps of said method including: determining a weighting factor in relation to each of selected roll stands for providing a predetermined portion of said roll opening correction to each selected roll stand, and controlling the roll opening of each of said selected roll stands in accordance with said correction and said weighting factor for that roll stand.
 10. The method of controlling the workpiece gauge leaving a rolling mill of claim 6 with said method including: determining said roll opening correction in relation to a predetermined weighting factor for applying a decreased correction to said first roll stand in accordance with the distance between said first roll stand and the last roll stand of said rolling mill.
 11. The gauge control system for a rolling mill of claim 1 with said correction being determined in accordance with a weighting factor in relation to a desired portion of said gauge correction that is to be applied to said first roll stand.
 12. A gauge control system for a rolling mill having at least a first roll stand and a second roll stand operative to reduce the gauge of first and second workpieces passed through said rolling mill and including a device for measuring the gauge deviation of said first workpiece leaving said second roll stand, said system comprising: means for determining a gauge correction for application to said first roll stand during the passage of said second workpiece in accordance with a predetermined relationship between said gauge deviation of said first workpiece, the operating speed of said first roll stand, the operating speed of said second roll stand and a weighting factor in relation to a desired portion of said determined gauge correction that is to be applied to said first roll stand, and means for controlling the roll opening of said first roll stand during the passage of said second workpiece in accordance with said gauge correction.
 13. A method of controlling the workpiece gauge leaving a rolling mill having at least a first roll stand operative to reduce the gauge of first and second workpieces passed through said rolling mill and in relation to a measured gauge deviation of said first workpiece leaving said rolling mill, the steps of said method comprising: determining a gauge correction for application to said first roll stand during the passage of said second workpiece in accordance with a predetermined relationship between the measured gauge deviation of said first workpiece, the workpiece movement from said rolling mill as compared to the workpiece movement from said first roll stand, determining a predetermined weighting factor in relation to said first roll stand, and controlling the operation of said first roll stand in accordance with said gauge correction and said weighting factor. 